In forming semiconductor devices, substrates typically are processed through a number of different pieces of equipment, including resist coating and developing tracks. As the substrates are processed through the equipment, particles from the equipment chucks can accumulate on the backsides of the substrates. These particles can cause problems in subsequent processing steps, including lithography or other imaging steps. Depth of focus in lithography is critical. The particles on the backside of a substrate can cause a substrate to bow beyond the depth of focus as vacuum is applied.
FIG. 1 and FIG. 2 can be referenced to better understand the above particle problem. FIG. 1 is a top view of a chuck 10 which is typical of those currently used in semiconductor substrate handling equipment, such as track systems. Chuck 10 includes a plurality of raised portions in the form of concentric rings 12, a vacuum port 14, and a plurality of recessed vacuum channels 16. A substrate (not shown) is positioned on the chuck, ideally centered, and a vacuum is applied through vacuum port 14. Segmented vacuum channels 18 connect the concentric vacuum channels 16, and enable a vacuum to be drawn with adjacent concentric rings 12 by a single, centralized vacuum port 14. Once vacuum is applied, the substrate is processed. For example, a resist is dispensed on the substrate and spun on the chuck to fully coat the front side of the substrate. During the spin cycle, particles of resist are generated and become suspended in the air around the wafer due to turbulence of air flow within a spinner cup of the spin track system. These particles eventually build up on the chuck, particularly during the period between removing the substrate from the chuck and positioning the next substrate on the chuck.
FIG. 2 is a representation of a typical particle distribution on the backside of a semiconductor substrate 20 when using the chuck illustrated in FIG. 1 in a resist coater. Particles 22 correspond to isolated particles which are not introduced as a result of using chuck 10, but simply are the result of particles landing on the backside of the substrate while the substrate is mounted on the chuck. Particles 24 correspond to the outer edge of the chuck, and particles 26 correspond to the inner concentric rings 12 of the chuck. (It is noted that in FIG. 2 the size of chuck 10 is smaller than as illustrated in FIG. 1. In practice, the substrate is likely to be quite a bit larger than the chuck holding the substrate.) As is evident from FIG. 2, the most dense particle distribution corresponds to the raised areas of the chuck (i.e. those portions of the chuck in direct contact with the substrate, such as raised portions 12 which separate adjacent concentric vacuum channels 16). In these areas, the particle density will likely lead to subsequent processing problems due to the severity of bowing of the substrate once mounted on another chuck.
One prior art attempt to solve the problem of backside particles uses an edge bead removal system. However, edge bead removal typically does not remove particles that are introduced onto the substrate by the chuck. In edge bead removal processes, typically a solvent is directed to the edge of the substrate to remove resist on or near the edge of the substrate. In some edge bead removal processes, the solvent is also directed to the backside of the substrate near the edge. However, when directing the solvent to the substrate backside, the area of the backside of the substrate in contact with the chuck and the area immediately surrounding the chuck cannot be exposed to the solvent and thus is not cleaned.
Another prior art attempt to solve the problem is to mismatch or offset the contact areas between the chuck and the substrate in subsequent processing steps. For example, the contact area pattern for a chuck in a resist coater is made to be different than the contact area pattern for a chuck in a stepper. Particles on the backside of the substrate as a result of resist coating are thus designed to lie within vacuum channels of the chuck in the stepper. However, even optical inspection methods cannot always confirm if the positioning of the particles are within a "safe" area for chucks used in subsequent processing or if the density of particles is acceptable.
Still another attempt to fix the particle problem is the use of pin chucks at processing steps subsequent to the processing step wherein the particles are introduced onto the backside of the substrate. Although the amount of surface contact between the pin chuck and the substrate is reduced, there still can be a location where the pin chuck contacts a particle on the backside of the wafer. Over time particles can accumulate on the pins and require cleaning. The cleaning of a pin chuck is difficult due to the number of pins. Cleaning of the pins can also affect pin planarity, leading to critical substrate focus shifts.
Particles on the backside of a substrate can lead to problems at processing steps other than lithography, including ion implantation and plasma etching steps. In the case of ion implantation, a substrate is heated by an ion beam that is being used to introduce dopant into the substrate. This heat can cause the resist to reticulate and make removal of the resist during subsequent steps, such as a plasma ash resist strip, nearly impossible. Within a plasma etching chamber, the particles can be transferred from one substrate to another. Eventually, particles are transferred out of the equipment and placed into a piece of processing equipment that is sensitive to contamination, such as a diffusion furnace. Furthermore, backside particles on a substrate can lead to non-uniform etching of the substrate due to temperature gradients across the substrate surfaces.
Therefore, a need exists to reduce the amount of particles being introduced onto the backside of a substrate from a chuck, particularly when liquids are being dispensed onto the substrate. A need further exists to reduce the probability that the particles will permanently remain on the substrate, particularly for more complex process flows that have upwards of twenty-five masking steps.